This paper is a contribution to the “theory” of the activity of using computers. It shows how some forms of expression used in current programming languages can be modelled in Church's λ-notation, and then describes a way of “interpreting” such expressions. This suggests a method, of analyzing the things computer users write, that applies to many different problem orientations and to different phases of the activity of using a computer. Also a technique is introduced by which the various composite information structures involved can be formally characterized in their essentials, without commitment to specific written or other representations.